This invention concerns an artificial satellite equipped with means of creating moments of both magnetic and aerodynamic origin, and a control process for such a satellite.
It is to be compared with the subject of the French patent whose application has been registered under No. 9304953 and which describes an artificial satellite equipped with aerodynamic orientation control surfaces and traveling in low or very low orbit, that is to say whose altitude, or apogee altitude if the orbit is elliptical, is less than 600 km in the case of the Earth. The invention is also applicable to other planets provided that such planets possess an atmosphere. In fact, the rarefied gases of the upper layers of the atmosphere, made up essentially of atomic oxygen for the Earth, may exert rotation moments on the satellite. The aerodynamic control surfaces, which exploit this circumstance, consist of turning panels which are oriented in order to oppose a cross-section of greater or lesser size to the flow or to supply it with an oblique and variable flight direction. It is possible to produce moments of any direction in function of the layout and orientation of the panels. In this way the orientation of the satellite is adjusted so as to maintain, against all disruptive influences, the pointing of instruments and detectors towards an objective, or of photovoltaic cells towards the sun.
The aerodynamic control surfaces have the advantage of economizing the ergol on board in relation to the thrusters or other means of the same type commonly used to move and orient the satellites, but they nevertheless consume this energy, directly for moving the said satellites and indirectly to compensate for the drag effect which they cause it is therefore desirable, while appreciating such means, to seek to reduce their number or attenuate their use.
The underlying idea of the invention therefore consists in combining the aerodynamic control surfaces with other means, namely magneto-couplers or magnetic torque generators supplied with renewable energy by photovoltaic cells and having the capacity to produce rotation moments in the presence of an ambient magnetic field such as the Earth""s magnetic field, according to the formula {overscore (C)}={overscore (M)}{circumflex over ( )}{overscore (B)} where {overscore (C)} is a torque in newton-meters, {overscore (M)} a magnetic moment in ampere-turns/square meters and {overscore (B)} a magnetic field in tesla. An unwelcome result of this formula is that the torques around the direction of the ambient magnetic field cannot be produced. It is for this reason that recourse will be made to aerodynamic control surfaces although they may also be used as complementary means of magnetic torques produced around other axes.
Other satellites are also known on which the magnetic torques are used, either in isolation or as a complement to orientation systems such as reaction wheels or momentum wheels when the said wheels reach saturation point, but it would seem that adjustable aerodynamic control surfaces have never been proposed except in the recent, previously mentioned patent application by the same author.
The aerodynamic control surfaces will produce torques and rotations around two of the principal axes of the satellite whose orientation will be chosen in function of the orbit position in relation to the magnetic field of the Earth or the planet under consideration. The magnetic couplers will produce moments around the three principal axes of the satellite. The orientation system will thus possess means which are concurrent but not superfluous inasmuch as the satellite traveling in orbit and pointed in an invariable direction will never have a fixed orientation in relation to the direction of the field, with the result that the magnetic torques will each in turn have their use, and the advantage derived from having recourse to them is such as to make them preferable to the control surfaces, despite the fact that some of them have only a slight effect.
To summarize, the invention concerns in its most general form an artificial satellite, characterized in that it comprises magnetic generators for producing rotation moments around three principal axes of the satellite, and turning external panels to supply aerodynamic rotation moments around two of the principal axes, depending on the type and orbit of the satellite in function of command laws associating the magnetic generators and the panels in order to induce them to produce variable moments. The invention also concerns a process which may be used for such a satellite in orbit around a planet and immersed in an atmosphere and a magnetic field, characterized in that it consists of periodically measuring or estimating the planet""s magnetic field in relation to the principal axes of the satellite, and of calculating the rotation moments to be produced around the principal axes by the magnetic generators and the external panels.